Digital communication relies on accurate timing; in this age of high bit rate digital communication, there is a large need for high frequency oscillators to time the high speed digital circuits. Furthermore, because of the degree of miniaturization prevalent in the industry, the need is particularly great for miniature oscillators manufacturable in low cost, very large scale integrated technology (VLSI) such as metal oxide semiconductor (MOS).
For the required accuracy to keep communication circuits synchronized, the oscillators may be controlled by a high Q resonator, common resonators include L-C tank circuits, tuned cavities, quartz crystals, and ceramic resonators. When a voltage controllable oscillator using a high Q resonator is used in a phase locked loop to match one frequency with another, its frequency must be variable over a reasonable range. This combination of requirements has proven hard to meet at low cost. The highest frequency at which an oscillator will operate is limited by the gain of the oscillator transistor; likewise, the frequency range through which an oscillator can be pulled is also limited by its gain. Unfortunately, the gain of semiconductor transistors manufactured in MOS is low compared, for example, to that of similar size transistors in bipolar technology. When the width to length ratio of an MOS transistor is increased to increase the gain, however, the gate-source and gate-drain capacitances of the transistor also increase. The resulting increased loading of the oscillator circuit offsets the increase in gain to result in only a small increase in maximum operating frequency, with substantial added current drain. Significant further reduction in effective gain of MOS transistors is caused by the body effect. Until our invention, therefore, the frequency range of mini oscillators in available VLSI technology has been severely limited.
An object of this invention is a mini oscillator that is manufacturable in VLSI technology and that has a significantly improved frequency range and a high pull range.
When oscillators are used in timing applications for digital circuits, a square wave with a duty cycle close to 50% is desirable to allow equal time for circuit action triggered by either the rising or the falling pulse edge. When oscillators are operated near their maximum frequency, however, the duty cycle is controlled by transistor parameters that tend to be asymmetrical. Conventional feedback circuitry may be used to force the duty cycle to 50%, but this can severely limit the oscillator frequency and is often a source of additional phase noise.
The duty cycle of an oscillator circuit embodying this invention can be readily controlled to approximately 50% without substantially limiting the maximum frequency of oscillation, and can be proportionally varied by an external signal over a chosen, controlled range.